Intraluminally implantable frames are being implanted in increasing numbers to treat a variety of conditions and are coming into greater use in a variety of fields. Frames implanted in vessels, ducts or channels of the human body can form part of a valve to regulate fluid flow within a body lumen or as scaffolding to maintain the patency of the vessel, duct or channel lumen. Implantable frames can also support a valve or valve leaflets for regulating fluid flow within a body lumen or for dilating a body lumen. One or more flexible valve leaflets can be attached to an implantable frame to form a medical device useful as an artificial valve. A variety of other implantable prostheses, such as stents, grafts and the like, also comprise an implantable frame placed within the body to improve the function of a body lumen.
The venous system includes a series of valves that function to assist the flow of blood returning to the heart. These natural valves are particularly important in the lower extremities to prevent blood from pooling in the lower legs and feet. Pooling of blood in the venous system may occur during certain situations, such as standing or sitting, when the weight of the column of blood in the vein can act to prevent positive blood flow toward the heart. This condition, commonly known as chronic venous insufficiency, is primarily found in individuals in which gradual dilation of the veins, thrombotic events, or other conditions prevent the leaflets of the native valves from closing properly. The failure of native valves to close properly can worsen, leading to significant leakage of retrograde flow such that the valve can become incompetent. Chronic venous insufficiency is a condition in which the symptoms can progress from painful edema and unsightly spider or varicose veins to skin ulcerations. Elevation of the feet and compression stocking can relieve symptoms, but do not treat the underlying disease. Untreated, the disease can impact the ability of individuals to perform in the workplace or maintain their normal lifestyle.
One promising approach to treating venous valve insufficiency includes the implantation of radially-expandable artificial valves placed using minimally-invasive techniques. Recently, the development of artificial and biological valves has been employed in an attempt to return normal pressure to the veins. These valves are generally designed to allow normal flow of blood back to the heart, while preventing retrograde flow. For example, U.S. Pat. No. 6,508,833 discloses a multiple-sided medical device comprising a closed frame of a single piece of wire or other resilient material and having a series of bends and interconnecting sides. A flexible covering of fabric or other flexible material may be attached to the frame to form an artificial valve. The flexible material utilized in these valves can comprised collagenous submucosa obtained from various animals, such as, pigs, cattle, and sheep. This material can be processed and preserved so as to be capable of inducing host tissue proliferation, remodeling, and regeneration of appropriate tissue structures, e.g., veins upon implantation in vivo (see, e.g., U.S. Pat. No. 6,485,723). The preparation of submucosal material is generally described in U.S. Pat. Nos. 4,902,508 and 5,554,389. The submucosal material can be prepared in large, flat sheets, which are subsequently cut and attached to a framing element, for example a stent, for deployment in a vein.
Dynamic fluctuations in the shape of the lumen of a body vessel, such as a vein, pose challenges to the design of implantable devices that conform to the interior shape of the body vessel. The shape of a lumen of a vein can undergo dramatic dynamic change as a result of varying blood flow velocities and volumes there through. Such dynamic change presents challenges for designing implantable intraluminal prosthetic devices that are compliant to the changing shape of the vein lumen. In addition, blood flow within a vein is intermittent and bidirectional, and subject to constant fluctuation in pressure and volume. These conditions in a vein present challenges to designing an implantable frame suitable for placement inside the vein. On one hand, an implantable frame lacking sufficient radial strength may fracture under repeated fluctuations of the vein diameter. On the other hand, an implantable frame with undesirably high levels of radial strength may lack flexibility and may damage the vein by failing to compress in response to normal fluctuations in the vein diameter. Likewise, an implantable frame with a high surface area contacting the interior wall of a vein may induce trauma in the vein wall, while an implantable frame with an insufficient surface area may lack sufficient durability. In addition, support frames are preferably configured to reduce or prevent stagnation of blood flow in regions of the body vessel proximate to the implanted frame, so as to mitigate or reduce incidence of thrombosis. Hence, what is needed is an intraluminally-placed medical device, such as an artificial valve or support frame, that is configured to withstand fluctuations in a body vessel without fracturing, to minimize the area of contact of the support frame with the wall of a body vessel and to create more desirable flow patterns around a valve within a body. For instance, a support may be configured to permit the circulation of blood or bodily fluids through a lumen and reduce the likelihood of stagnation and the potential thrombotic conditions near the support frame. These conditions, such as more turbulent flow, increased velocity of flow, larger and/or more numerous vortices, other factors, or a combination of the above, can mitigate the incidence of thrombosis formation near the implantable medical device.
There remains a need, therefore, for prosthetic valves having a support frame configured with a radial strength to maintain patency of a body vessel while supporting a means for regulating fluid within the body vessel and minimizing irritation to the body vessel after implantation.